Now we know that Brexit means Brexit, even if parliament votes against the deal negotiated by the Prime Minister. The stark truth which has emerged over the last 24 hours highlights clear evidence that we should all be worried about the future of the UK's parliamentary sovereignty, which many are so keen to wrestle back from Brussels. And that matters hugely for the future of the UK's environment.

Air pollution in Britain is a pervasive killer, but it is also intangible and invisible. You can rarely see it, you can't always smell it, and it is unlikely you will ever hear it. It is even less likely, apparently, that you will have heard about it if you happen to be a Conservative Party minister.

DEFRA, the UK's environment ministry, was taken to the High Court by international environmental law firm ClientEarth over its failure to meet legal limits on air quality in 17 regions and cities across Britain. Whilst David Cameron would never be confused with Dan Quayle in terms of his environmental knowledge, it is time for him to match his awareness of the issues with the leadership we need to tackle this public health crisis. After all, don't we all have the right to take a stroll down our local high street in the knowledge it won't take years off our lives?